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We demonstrate a high speed GeSi electro-absorption (EA) modulator monolithically integrated on 3 µm silicon-on-insulator (SOI) waveguide. The demonstrated device has a compact active region of 1.0 × 55 μm2, an insertion loss of 5 dB and an extinction ratio of 6 dB at wavelength of 1550 nm. The modulator has a broad operating wavelength range of 35 nm and a 3 dB bandwidth of 40.7 GHz at 2.8 V reverse bias. This compact and energy efficient modulator is a key building block for optical interconnection applications.
©2012 Optical Society of America
1. Introduction
Silicon photonics has been the target of significant research and development work in the past few years [1–5] due to its electronics integration capability and the potential for the fabrication of low power consumption optical components using this technology. Many silicon based photonics components, such as high-speed modulators [6–18], photodetectors and WDM filters have been demonstrated. Among these, silicon based modulators still have many challenges that need to be overcome in order to achieve low power, broadband and high speed operation. Owing to the weak electro-optical effect of silicon, most demonstrated waveguide based silicon modulators utilize the free carrier dispersion effect with a Mach-Zehnder interferometer (MZI) or a ring resonator. The silicon MZI modulator is usually a few millimeters long and hence consumes tens or hundreds of milliwatts. The silicon ring modulator has low power consumption, but the working wavelength is very sensitive to fabrication imperfections and may need active tuning or even an additional wavelength locker. In the GeSi or Ge material system, which is CMOS compatible, there is a strong electro-absorption (EA) effect which is intrinsically an ultrafast process suitable for very high-speed modulation. The EA effect is known as the Franz-Keldysh (FK) effect in bulk semiconductors and the quantum-confined Stark effect (QCSE) in quantum-well (QW) structures. The EA modulator is a good candidate providing compactness, high speed, low power and broad band operation. Recently, Liu et al [12] demonstrated a GeSi FK modulator integrated on a submicron silicon waveguide with 1.2 GHz modulation speed. High speed Ge FK modulators that work around 1620 nm have been demonstrated on 3 µm silicon waveguides [18]. However, to the best of our knowledge, a high speed GeSi FK modulator that has a working wavelength in the C band has not been demonstrated. In this paper, we demonstrate an EA modulator that can operate at a wavelength near 1550 nm by selective growth of GeSi material on an SOI wafer with a silicon composition of 0.7%. The device is integrated with a 3 μm thick SOI waveguide using a horizontal p-i-n structure in GeSi material. The demonstrated device has a very compact active region of 1.0 × 55 µm2 and a working wavelength range from 1546 nm to 1581 nm. The measured insertion loss of the device is 5.0 dB and the extinction ratio is 6.0 dB at a wavelength of 1550 nm with a swing voltage of 2.8 V. The 3 dB bandwidth measurement shows that the device can operate at 40.7 GHz with 2.8 V reverse bias. A clear eye-opening at a transmission rate of 28 Gbps demonstrates the capability of high-bit-rate large signal modulation.
2. Device design and fabrication
The design of high speed devices like FK modulators is very challenging on the 3 µm waveguide platform because the larger scale tends to result in high capacitances, or a weak electrical field that limits the operation speed or increases the power consumption. To overcome these challenges, we have proposed and demonstrated a novel Ge horizontal p-i-n structure [4, 18]. Here we use a similar structure but with selective growth of GeSi instead of pure Ge. The FK modulator and a cross-section of the active p-i-n diode region are schematically shown in Fig. 1 . As the light propagates from the ridge SOI waveguide to the FK modulator region, the light is absorbed in the active GeSi region. The amount of absorption depends on the applied voltage across the p-i-n junction. This horizontal p-i-n configuration enables a very narrow intrinsic GeSi region, hence reducing the voltage swing required to achieve a high extinction ratio.
The modulator was fabricated on six inch SOI wafers with 0.4 µm thick buried oxide (BOX) and a 3 µm thick silicon epitaxial layer. First, the silicon recess region was formed by etching the silicon layer down to a thin residual thickness above the BOX layer. Then, GeSi with an estimated silicon composition of 0.7% was selectively grown inside the silicon recess region in order to ensure C-Band operation [12]. The silicon ridge waveguides and GeSi waveguides were formed by dry etch. Boron and phosphorus were implanted in the sidewalls and slabs of the GeSi waveguide to form a horizontal p-i-n junction and p-type and n-type ohmic contact areas. After rapid thermal annealing (RTA) dopant activation, the Ti/Al metal stack was deposited and dry etched to form p-type and n-type metal contacts. More details on device fabrication can be found in references [4, 18]. A cross-section scanning electron microscopy (SEM) image of the final fabricated device is shown in Fig. 2 . During fabrication, the GeSi was under-etched, leaving a GeSi slab thickness of around 0.55 µm. This thick GeSi slab causes excess transition loss between the silicon waveguide and the GeSi waveguide [18].
3. Measurement results
The leakage currents at reverse biases of −0.5V, −1.0 V and −1.4 V were measured at 80 μA,680 μA and 1.6 mA, respectively, for a modulator with an active area of 1.0 μm x 55 μm. Figure 3 shows the measured I-V characteristics of the fabricated devices. The leakage current of the modulator is high. One possible reason for this high leakage current is a high defect density at the interface between GeSi and Si as indicated by transmission electron microscopy (TEM) images. Another possible reason is the presence of metal residue following Ti/Al metal dry etching. The high leakage current can be significantly reduced by optimization of the GeSi material growth and post annealing conditions, or by over etch of the Ti/Al to remove the metal residue.
The fabricated modulator devices were measured using an automated measurement system. The light from a tunable laser was coupled to the device through a lensed fiber. The light from the output waveguide was also coupled to a lensed fiber and then to a photodetector. Figure 4(a) shows the measured transmission spectra for different applied reverse voltages across the p-i-n junction. Because the devices have high leakage currents for reverse biases greater than 1 V, the devices will heat up when large reverse biases are applied. As a result, the measured transmission spectra shown in Fig. 4(a) include both FK and thermal effects. For the FK effect, the absorption coefficient increases with the applied electric field when the photon energy is below band gap because of tunneling process of electrons and holes, the absorption coefficient oscillates as a function of photon energy when the photo energy is above the band gap. When the photon energy is very close to band gap, the absorption coefficient is almost not change with applied electric field and the absorption spectra for different applied voltages will cross at band gap. The FK effect tilts the GeSi band edge to longer wavelengths with increasing applied electrical field, while the thermal effect shifts the GeSi band edge to longer wavelengths with increasing temperature. When the bias is below 0.5 V, only the FK effect is present since the leakage current is very low and there is almost no heating power applied to the device. The transmission spectra for 0 V and 0.5V biases overlap at a wavelength of 1511.5 nm. By blue-shifting the transmission curves for biases greater than 0.5 V so that the spectra all overlap at 1511.5 nm, the slow thermal effect can be removed from the measured transmission spectra. Figure 4(b) shows the modified transmission spectra after the thermal effects are removed using this method. Figure 5 shows the insertion loss and the extinction ratio in the wavelength range from 1500 nm to 1600 nm. The extinction ratios shown in Fig. 5 are representative of the response of a practical device as this fig. excludes the thermal induced extinction ratio which will not be present when the device operates at high speed, since the thermal response is very slow in comparison to the modulation speed and the device will reach a constant operating temperature.
Fig. 4 (a) Measured transmission spectra for different applied reverse voltages. (b) Transmission spectra for different applied reverse voltage after thermal effects are removed.
The insertion loss of the device is 5 dB at 1550 nm. The principal contributors to this loss are the GeSi absorption loss that is present without the applied voltage, and the mode mismatch loss between the silicon waveguide and the GeSi waveguide. In Fig. 6(a) , the transition loss is calculated by using the beam propagation (BPM) method based on waveguide dimensions measured from SEM images of fabricated devices, indicating a GeSi waveguide width of 1.0 μm, and GeSi slab width of 0.55 μm. The simulated transition loss is around 1.0 dB due to the unintentionally thicker GeSi slab of 0.55 μm. The transition loss of 1.0 dB would be significant decreased if the GeSi slab thickness was less than 0.3 μm. By excluding the transition loss, the Δα/α (ratio between extinction ratio and the GeSi absorption loss) of the EA modulator can be calculated based on the measured extinction ratio and insertion loss of the device as shown in Fig. 5. Figure 6(b) shows the extracted Δα/α dependence on the electrical field at 1550 nm, calculated by using an estimated total boron and phosphorus doping depth of 0.25 μm at the GeSi waveguide sidewall. Δα/α is close to 1.6 at an electric field of 42 kV/cm which is close to the estimated value in [13]. Δα/α is close to 3.1 at an electric field of 75 kV/cm predicted by a linear fitting of the measured values which is close to the experiment value reported in [19]. The performance of the GeSi FK modulator can be further improved by reducing the transition loss and increasing the electrical field by reducing the GeSi waveguide width.
Fig. 6 (a) BPM simulation of the transition loss between GeSi and Si waveguides excluding GeSi absorption. (b) Measured Δα/α for different electrical field at 1550 nm.
The link power penalty of a modulator can be defined as OMA/(2Pin) = (Pout(1) - Pout(0))/ /(2Pin), where OMA is the optical modulation amplitude, Pin is the input optical power to the modulator, Pout(1) and Pout(0) are the high level and low level of the output optical powers after the modulator, respectively. The link power penalty of a modulator includes the insertion loss, average optical power loss and limited extinction ratio penalties. It is a more accurate figure of merit value to measure the performance of a modulator. Figure 7 shows the link power penalty for different reverse voltage based on the insertion losses and extinction ratios shown in Fig. 5. The 1 dB wavelength bandwidth of the link power penalty is calculated as about 35 nm from 1546 nm to 1581 nm with the minimum link penalty of 8.5 dB. The demonstrated modulator has an insertion loss of 2.9 dB to 5.7 dB, and an extinction ratio of 2.6 dB to 6.6 dB in this working wavelength range.
The temperature dependence of the transmission spectra of the GeSi FK modulator was measured by using a TEC temperature controller under the FK modulator die. The measured spectral shift to longer wavelengths with increased temperature is shown in Fig. 8(a) . Figure 8(b) shows the wavelength at −10 dB transmission loss over the temperature range from 30 °C to 70 °C. The band edge shift coefficient of 0.76 nm/°C is obtained by linear fitting of the measured data. The working wavelength range can be further increased by an efficient local thermal heater as the device area is only 0.55 μm2.
Fig. 8 (a) Measured transmission spectra for different temperature. (b) Measured wavelength at −10 dB transmission for different temperature.
The high speed bandwidth of the demonstrated FK modulator device was measured by a vector network component analyzer. The high-speed RF signal and DC bias voltage were applied to the FK modulator device through a bias-tee and a high-speed probe. The modulated light signal was coupled to a lensed fiber and amplified by an erbium doped fiber amplifier (EDFA), then through a tunable filter to reduce the amplified spontaneous emission noise of the EDFA. The amplified optical signal was detected by a commercial high-speed photodetector and then fed into a network analyzer. The RF system was calibrated in advance to factor out the effects of the cable, the bias-tee, and the photodetector response. Figure 9 shows the normalized frequency response for the FK modulator with various reverse bias voltages. The measured optical bandwidths were 20.6 GHz, 31.6 GHz and 40.7 GHz at biases of −1 V, −1.9 V and −2.8 V, respectively. The 3dB bandwidth of EA modulators is only limited by the RC time constant of the device [12, 18]. The width of the intrinsic region of the p-i-n junction increase with higher reverse bias due to the depletion. This makes the capacitance smaller so the modulator speed increases with higher reverse bias.
To demonstrate large signal high-speed modulation, a pseudorandom binary sequence (PRBS) signal with a (231-1) pattern length at a 28 Gbps transmission rate was set to swing from −0.7 V to + 0.7 V and then combined with a 1.4 V DC bias using a bias tee. The combined signal was then connected to the modulator through an electrical probe. The actual modulation voltage applied to the p-i-n diode is approximately from 0 V to 2.8 V as the AC signal is almost totally reflected by the capacitive p-i-n diode. The modulated light signal was amplified by an EDFA and coupled into a commercial photodetector attached to a digital communication analyzer. The eye-diagrams at 1550 nm wavelength were measured and are shown in Fig. 10 . A clear eye opening with a dynamic extinction ratio of 5.9 dB is achieved with 2.8 V swing voltage.
The junction capacitance of the EA modulator is estimated at 30 fF based on the p-i-n junction dimensions which have length of 55 µm, a height of 2.5 µm and an intrinsic width of 0.75 µm. The average energy consumption per bit for the dynamic modulation is given by energy/bit = 1/4CV2pp, where C is the junction capacitance, and Vpp is the swing voltage. It is calculated that the energy/bit for the demonstrated EA modulator at a Vpp of 2.8 V bias is about 60 fJ/bit. The dynamic power consumption is only 1.7 mW with 2.8 V swing voltage for a 28 Gbps data transmission rate. The photo-generated currents also contribute to the total power consumption. The responsivity of the EA modulator is measured at around 0.6 A/W, and the input optical power to the modulator is less than 6 mW for most applications. The total power consumption even including the photocurrent is about 6.7 mW for a 28 Gbps signal modulation. The high leakage current can be significantly reduced by process optimization, with which the leakage current caused additional power consumption can be negligible. It is also very important that the EA modulator can operate with a low swing voltage of 2.8 V, so a low power GeSi bipolar driver can be used. It is also possible that the GeSi EA modulator can use CMOS driver with swing voltage less than 2 V by reducing the GeSi waveguide width and optimizing the sidewall doping profiles.
4. Conclusion
In conclusion, we have demonstrated the first high speed GeSi EA modulator integrated with a 3 µm SOI waveguide with a working wavelength near 1550 nm based on the Franz-Keldysh (FK) effect. The demonstrated modulator has a 3 dB bandwidth of 40.7 GHz and a broad operating wavelength range of 35 nm near 1550 nm. The demonstrated modulator has very compact size, high speed, low power and broadband operation. This GeSi EA modulator can readily be integrated into a monolithic silicon photonics transmitter for terabit data transmission applications.
Acknowledgment
This material is based upon work supported, in part, by DARPA under Agreement No. HR0011-08-09-0001. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. The authors thank Dr. Jagdeep Shah of DARPA MTO for his inspiration and support of this program. Approved for public release. Distribution Unlimited.
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